For automobile skeleton components (hereafter, also referred to simply as “skeleton components”) constituting a body of an automobile, efforts have been made to promote weight reduction and functional enhancement (for example, improvement of anti-collision performance). For that purpose, a tailored blank is used as the starting material for a skeleton component. The tailored blank is made up of a plurality of metal sheets integrated by being joined (for example, butt-welded) together, in which the plurality of metal sheets are different from each other in tensile strength, sheet thicknesses, and the like. Hereinafter, such a tailored blank is also referred to as a TWB. A press-formed product is obtained by press-working a TWB. A press-formed product is subjected, as needed, to trimming, restriking or the like, thereby being finished into a desired shape.
For example, a front pillar and a side sill are each a complex body of skeleton components. The front pillar is disposed on a fore side of a vehicle body, and extends vertically. The side sill is disposed in a lower portion of the vehicle body, and extends in a fore-to-aft direction. A lower end section of the front pillar and a fore end section of the side sill are coupled to each other. Here, some structures of the front pillar may adopt a structure which is divided into upper and lower sections. In this case, the upper section is called as a front pillar upper, and the lower section as a front pillar lower. A lower end section of the front pillar upper and an upper end section of the front pillar lower are coupled to each other.
The front pillar lower includes, as skeleton components, for example, a front pillar lower-outer (hereafter, also referred to simply as an “outer”), a front pillar lower-inner (hereafter, also referred to simply as an “inner”), and a front pillar lower-reinforcement (hereafter, also referred to simply as a “reinforcement”). The outer is disposed on the outer side in the vehicle width direction. The inner is disposed on the inner side in the vehicle width direction. The reinforcement is disposed between the outer and the inner. Among those, the outer is curved in an L-shape along the longitudinal direction, and has a hat-shaped cross section over the entire range in the longitudinal direction. Typically, the outer is a press-formed product.
FIGS. 1A and 1B are schematic diagrams to show an example of a front pillar lower-outer which is a press-formed product. Of these figures, FIG. 1A shows a plan view, and FIG. 1B shows an A-A cross sectional view of FIG. 1A. Note that, to help understanding of shape, the side to be coupled to the side sill is designated by a symbol “S”, and the side to be coupled to the front pillar upper is designated by a symbol “U”.
As shown in FIG. 1A, the front pillar lower-outer 10 includes a curved region (see an area surrounded by a two-dot chain line in FIG. 1A) 13 which is curved in an L-shape along the longitudinal direction, and a first region 11 and a second region 12, which are respectively connected to both ends of the curved region 13. The first region 11 extends in a straight fashion from the curved region 13 rearwardly in the travelling direction of an automobile to be coupled to the side sill. The second region 12 extends in a straight fashion upwardly from the curved region 13 to be coupled to the front pillar upper.
As shown in FIG. 1B, the cross sectional shape of the outer 10 is a hat shape over the entire range in the longitudinal direction from an end to be coupled to the front pillar upper to an end to be coupled to the side sill. Therefore, each of the curved region 13, the first region 11 and the second region 12, which constitute the outer 10, includes a top plate section 10a, a first vertical wall section 10b, a second vertical wall section 10c, a first flange section 10d, and a second flange section 10e. The first vertical wall section 10b is connected with the entire length of the side forming the inner side of curve of the both side sections of the top plate section 10a. The second vertical wall section 10c is connected with the entire length of the side forming the outer side of curve of the both side sections of the top plate section 10a. The first flange section 10d is connected with the first vertical wall section 10b. The second flange section 10e is connected with the second vertical wall section 10c. 
It is possible to use a TWB for the production of such front pillar lower-outer 10. Regarding the method for shaping a press-formed product from the TWB, the following conventional techniques are available.
Japanese Patent Application Publication No. 2006-198672 (Patent Literature 1) discloses a technique to mitigate the load acting on the vicinity of a weld line of a TWB at the time of press working. In this technique, the TWB is provided with a cutout at a location slightly apart from the weld line. Patent Literature 1 describes that at the time of press working, strain which occurs in the vicinity of the weld line is dispersed by the cutout, thereby improving formability of the formed product.
Japanese Patent Application Publication No. 2001-1062 (Patent Literature 2) discloses a technique for applying press working on a TWB which is made up of two metal sheets each having a different tensile strength and a sheet thickness. In this technique, a weld line of the TWB is disposed on a portion where a gradient of strain would occur when a single metal sheet, which is not a TWB, is press worked. Then, a metal sheet having a higher strength is disposed on the side of larger strain, and a metal sheet having a lower strength is disposed on the side of smaller strain. As a result of this, strain will be reduced in press working such as deep drawing, bulging and the like. Patent Literature 2 describes that, as a result of that, cracking of the base metal which occurs in the metal sheet on the lower strength side is suppressed, thus improving the formability of formed product.
Japanese Patent Application Publication No. 2002-20854 (Patent Literature 3) discloses a technique to apply press working on a TWB which is made up of two metal sheets having similar levels of tensile strength and ductility. In this technique, a specific region in a formed product obtained by press working is subjected to a heat treatment such as nitriding, thereby strengthening the specific region. Patent Literature 3 describes that since deformation resistance of the metal sheet is uniform at the time of press working before the heat treatment, the formability of the formed product is improved.